Re: Cohen OM1556 Mic Preamplifier Schematic

Were you aware that the pdf referenced in the OP is a drawing of Greame's original preamp that incorporated the OM1556 hybrid he also designed?

The point is the Cohen AES paper and its schematics leave out a lot of really important stuff like phantom power protection.
The Cohen AES paper should not be taken literally.
For instance: Cohen shows no phantom power rail steering bridges in his AES paper.
If you look at the pdf those diodes are shown externally.
The circuitry outside the hybrid is just as important.

Somewhere in the archive is an OM1556 datasheet.
It may have a complete schematic of the hybrid internals.

I took issue many years ago with the topology being called "new" by Cohen in 1984 when it had been shown by Demrow in 1969.

Cohen combined a great many things done earlier. In an AES presentation he did, which the drawing above is from, he explained what "new" meant.

Re: Cohen OM1556 Mic Preamplifier Schematic

Posted: Sun Mar 10, 2019 3:21 pm

by terkio

The pdf you publish here, is great. I was not aware of this document from Cohen.
Indeed he missed phantom protection asking for 10 ohm serie resistors and diodes 3A surge capable.
I have seen Schotky diodes are best, here.
I see Cohen did not miss the peak Led; Do you think his circuit is good or nowdays we have better ?
Groner missed the peak Led.
Do you like on board phasing with a relay ? Do you recommend a part that will be reliable and THD friendly.
I think an onboard peak Led is a must, I woul like an onboard Vu meter driver too.

Re: Cohen OM1556 Mic Preamplifier Schematic

Posted: Sun Mar 10, 2019 4:23 pm

by mediatechnology

I'm not sure Cohen missed phantom protection.
He likely left them off the hybrid because they are big.
He put them on the PCB.

I like the glass-passivated 1N4004 series.
Vishay make them.
They have very low leakage which some Schotky diodes don't.

I have a whole thread on peak flashers.

Relays for polarity reverse work for me.

Re: Cohen OM1556 Mic Preamplifier Schematic

Posted: Sun Mar 10, 2019 5:13 pm

by terkio

I'll forget about Schotky, there is no real need for this, here. 1N4004 is fine 30A surge capable.
Do you have a favorite relay to switch line level audio ?

Re: Cohen OM1556 Mic Preamplifier Schematic

Posted: Thu Mar 14, 2019 11:13 am

by terkio

An after thought about my statement: "A peak led is a must on a preamplifier board".
When not implemented on the preamplifier boards, adding peak led and vu meter drivers afterward gives a not so clean chassis wiring.
OTOH, there are many variants about what users like to add or not.
To keep modularity and allow a clean chassis wiring:
I think it would be convenient to have on a preamplifier board a way to piggy back such an additionnal board.
So I would like a preamp pcb with the footprint for a hat connector giving +15v 0v -15v out+ out-
Then a user could plug on wathever monitoring stuff he likes.

Re: Cohen OM1556 Mic Preamplifier Schematic

Posted: Thu Mar 14, 2019 2:15 pm

by mediatechnology

I agree about the LED separate.

A simple LED winky board or a 5 to 10 step level meter as an outboard option is better and can be used in a lot of other places in addition to the preamp.

I've been wanting to do a simple gain reduction meter to replace the ones we had for the Pico Compressor using the LM3914 and make it so it could also be used as a simple level meter.

WRT to relays the Panasonic DS2Y low-signal relays seem to work pretty well.